1. Technical Field
The present invention relates to a method of mass producing laser diode units, each including a planar submount and laser diode mounted thereon, by using a high power pulsed laser operative to ablate the desired surface regions of the submount's metalized layer in a time- and cost-effective manner without formation of burrs.
2. Prior Art
High power semiconductor lasers have broad applications in various fields including, among others, military and industry. The rapid progress achieved in manufacturing semiconductor devices may be, in part, attributed to a planar technology in accordance with which a one-piece substrate is divided into a plurality of semiconductor devices by using masks made of photoresist.
The advances in high power laser diodes can be generally attributed to improvements of diode lasers performance and optimization of packaging architectures all based on the planar technology. The major characteristics of high power lasers, such as maximum useful output power, wavelength, lifetime are not only limited by the diode or semiconductor structure itself, but also strongly by the quality of the package including configuration and fabrication methods of heat sinks or sub-mounts. As known, the packaging process contributes strongly, about more than 50%, to the production costs of a high power diode laser. Accordingly, the laser diode package must be cost efficient. Needless to say, packaging techniques, including manufacturing of bases, are currently a very active area of research and development.
It is well known that both operating characteristic and longevity of laser diodes are strongly affected by the junction temperature. Edge emitting laser diodes operating at high current require a heat spreader to be placed between the device active region and the metal carrier package. When arranged in high densities, heat dissipation becomes even more critical. To minimize the severity of this problem, typically thin film metals with high thermal conductivity are used for heat sinks. Materials have to be thoroughly selected and combined so as to provide the desired topology and a combination of metals has to be thoroughly designed. Thus, along with a cost effective packaging technique, as power density increases, high reliability submounts should be configured to safeguard the stability of the active device, which is sensitive to changes in temperature.
Referring to FIG. 1, a typical process for fabricating submounts includes forming a base 10. The latter may be configured with a substrate carrier 12 made, as a rule, from thermo-conductive ceramic material, such as Beryllium oxide (BeO) or aluminum nitride (AlN). Further, a metal sub-layer 14 is plated on substrate 12, and a top metal sub-layer 18 is deposited atop sub-metal layer 14. The metal sub-layers, in combination, are configured to spread heat towards carrier 12 and provide a diffusion barrier. The equidistantly spaced soldering strips 16, coupling configuration 10 to laser diodes, which are provided after base 10 is divided into a plurality of submounts 25, as disclosed below, are applied to top metal layer 18 and typically made of Gold/Tin alloys (“AuSn”). The configuration 10 is then processed to have a plurality of insulation grooves 20 between electrical contacts of opposite polarity. Thereafter, base 10 is cut into a plurality of uniform submounts 25 along cutting lines 22. Finally, laser diodes are soldered to respective submounts.
Before cutting base 10 into submounts by a cutting saw, metal layer 14 is to be removed along cutting lines 22 and along isolating grooves 20 in respective regions A and B. Otherwise, a plurality of burrs can be formed while a saw (not shown) cuts configuration into submounts 25 which is unacceptable since it may affect the desired positioning of a laser diode or chip 24.
The removal of metal layers 14 and 18 is realized by photolithography and includes the use of photomasks made from photosensitive material or photoresist. The mask is applied to the surface and processed so that photoresist image is formed on the surface of the metal layer. To transfer this image into this layer, typically, two conventional etching methods are used: wet etching and ion milling. The wet etching is fast and, therefore, cost-efficient. However, during this process, because multiple metal sub-layers melt at different temperatures, resulting cutting edges are not planar which eventually leads to an angled position of laser diodes 24 in which one edge, for example, emitting edge extends in a plane higher than that of the opposite diode's end. The angled position may critically affect the operation of the diode. Yet another undesirable consequence is the formation of undercuts. The latter, in turn, detrimentally affects further alignment operations. The ion dry etching can provide sharp, planar vertical edges. However, this technique is slow. For example, etching 15-30 micron metal layer, typically takes about thirty (30) hours. In mass production such a long process is unacceptably expensive.
In both techniques, when photoresist is applied to a relatively porous metal surface, it soils the surface. The cleaning of the surface may not be entirely successful. If the surface is still not completely free from photoresist, subsequent technological procedures may not be effective. For example, a soldering material may interact with the photoresist which detrimentally affects the coupling between a substrate and laser diode.
Common to the above-discussed techniques, it should be noted that it is very difficult to control its parameters once the metal removal process starts. For example, removing metal material for subsequent cutting of a one-piece planar base into a plurality of submounts does not always require the removal of all metal sub-layers. In contrast, forming an isolation groove requires ablating metal layers in their entirety. The impossibility of manipulating parameters associated with both techniques during the photolithographic process certainly contributes to relatively high costs associated with the production of laser diode units.
A need therefore exists for an improved method of manufacturing submounts for laser diodes.
A further need exists for a cost effective, quality oriented method of removing metal from the surface of a multilayer configuration used as a base for laser diodes.